You just do the same as when rendering in a "normal" windowed mode.

Just set the HWND of the pane (GetPane) inside the presentation parameters. And act on resizing.

If you are looking to render to each of the frames in the SplitterWnd, you may not want to create a new device for each View. You'll want to create a Swap Chain for each View. This lets you share one Direct3D8 Device with each view you want to render. You can create a new Swap Chain by calling CreateAdditionSwapChain() on your Direct3D8 Device.

The Swap Chain acts like a rendering surface, so you can access the back-buffer and set it as your rendering target and draw your scene. Hope this helps.